Existing 2G to 3G cell reselection is based on a threshold on UTRAN (universal terrestrial radio access network) CPICH (common pilot channel) Ec/No (received energy per PN chip from a CPICH to a total received power spectral density at an antenna connector of a mobile station) and on a UTRAN CPICH RSCP (received signal code power) level relative to the GERAN (GSM/EDGE radio access network) serving cell RXLEV (received signal level). In addition, cell reselection towards UTRAN may be forbidden if the UTRAN target cell suitability criteria S (Srxlev>0 and Squal>0) are not fulfilled (a reselection may still be triggered).
TeliaSonera has suggested a signaling change based improvement with a new threshold value for the UTRAN CPICH RSCP to control reselection from GERAN to UTRAN in addition to existing parameters (see (1) Change Request 45.008 CR 205 #GPP TSG-GERAN Meeting #19 Cancun, Mexico, 19-23 Apr. 2004 Tdoc GP-040586, (2) “Background for WCDMA uplink best characterized by CPICH RSCP and downlink by CPICH Ec/Io,” Tdoc GP-040585, and (3) Proposal document “Need to study for Cell Re-selection specification changes,” 10.06.04 by Ari-Pekka Salovaara).
Currently there is no threshold for the minimum acceptable UTRAN CPICH RSCP signaled in GSM (global system for mobile communications) when reselecting a UTRAN cell from GERAN. The consequence is that it is impossible to apply an aggressive reselection behavior where the mobile station would reselect an available UTRAN cell whenever the UTRAN cell is suitable (plus a practical hysteresis to avoid ping-pong).
More specifically if the reselection is controlled with the existing Ec/No threshold value only (by setting the RSCP versus RXLEV offset criteria to “minus infinity”, i.e., allowing reselection irrespective of the serving cell RSSI [received signal strength indicator] versus the target cell RSCP), the mobile may reselect an unloaded UTRAN cell from too far away where the uplink fails (specifically if that UTRAN cell is at the edge of the UTRAN coverage). On the other hand, while setting the Ec/No threshold sufficiently high (to guarantee an uplink access), the mobile station may not be able to access a loaded UTRAN cell at all or at least the mobile station would need to go far closer to the target UTRAN site than intended by network planning.
The origin of the problem, as explained in the TeliaSonera documents mentioned above, is the fact that the UTRAN CPICH Ec/No varies a lot depending on the load of the UTRAN cell and especially for a cell at the edge of UTRAN coverage. This makes it impossible to set the reselection occurring at a point where the uplink is guaranteed to be operational while at the same time making the mobile station reselect the UTRAN cell whenever both uplink and downlink are operational (basically when both UTRAN CPICH RSCP and Ec/No are at a sufficient level). In the suitability criterion of UTRAN a minimum CPICH RSCP based threshold is included.